1. Field of the Invention
This invention relates to ultrasonic motors and, more particularly, to ultrasonic motors having an improved adhesive layer between an electromechanical conversion element and an elastic body.
2. Description of the Related Art
A traveling wave ultrasonic motor is disclosed in JP-B-1-17354 (Japanese Examined Patent Publication 1-17354) which utilizes the expansion and contraction of an electromechanical conversion element, such as a piezoelectric body, to convert electrical energy into mechanical motion. A traveling wave is generated in the drive surface of an elastic body and a moving body in pressure contact with this drive surface is driven by the traveling wave.
In the device of JP-B-1-17354, the elastic body is integrally joined to the piezoelectric body and the elastic body is excited by the vibration of the piezoelectric body.
The method of joining the elastic body and the piezoelectric body is a weld deposition or an adhesive method. In many instances, the piezoelectric body Curie point is generally 350.degree. C. or less since exposure to a high temperature state is not possible in practice.
However, use of an adhesion layer to join the elastic body and the piezoelectric body can cause unwanted attenuation of the vibration of the elastic body. This attenuation of the vibration of the elastic body can be reduced by providing a relatively thin adhesive layer, as described in Nikei Mechanical, December 1990, Extra Issue, "Motor Applications."
However, as recognized by the present invention, the adhesive layer joining the elastic body and the piezoelectric body is not simply a structural junction at a surface; it also serves as a functional junction to propagate vibration. Therefore, the present invention recognizes that, when the adhesive layer is made relatively thin, the properties of the adhesive layer cause an attenuation of the vibration of the piezoelectric layer. This results in an insufficient starting torque of the ultrasonic motor. In addition, the power consumption is increased, the drive efficiency is reduced and the performance of the ultrasonic motor falls off.
Additionally, the elastic body and piezoelectric body differ greatly in thermal expansion coefficients due to the nature of the different material characteristics. The differing amounts of expansion due to temperature changes causes a deformation of the motor stator. In order to prevent this phenomenon, an elastic body and an electrostrictive element of about the same thermal expansion coefficient was proposed in JP-A-60-62883 (Japanese Laid-Open Patent Publication 60-62883).
However, while the thermal expansion coefficients of the elastic body and the piezoelectric body may be the same in the device as described in JP-A-60-62883, the present invention recognizes that this method of reducing the effect of temperature changes does not consider that an adhesive layer exists between the elastic body and the piezoelectric body. The thermal expansion coefficient of the adhesive layer can typically be one to two orders of magnitude greater than the thermal expansion coefficient of a metal or piezoelectric body.
Also, because the adhesive layer used to join the elastic body and piezoelectric body has an additional function of reliably transmitting the vibration of the piezoelectric body to the elastic body, the present invention recognizes that it is preferable to use an adhesive which is comparatively hard after setting. Moreover, there are many adhesives which set by chemical reaction and generally shrink after setting. In the event that such an adhesive shrinks, the present invention recognizes that material which is adhered is pulled in the direction of shrinkage. As a result of a shrinking adhesive which has become relatively hard after setting, the adhesive absorbs tensile stress of the adhered material and the adhered material becomes deformed.